Paleogeography of the Tokyo and Nakagawa Lowlands since the Last Glacial Maximum

Abstract

 This paper reconstructs the paleogeography of the Tokyo and Nakagawa Lowlands, Kanto Plain, central Japan since the Last Glacial Maximum (LGM) on the basis of 18 sediment cores and 467 radiocarbon dates obtained from the lowlands. Fourteen sedimentary facies were identified from the sediment cores, and paleo water-depths were calculated from sediment accumulation and sea-level curves in the lowlands. The paleogeography of the Tokyo and Nakagawa Lowlands has been controlled by basement morphology, tidal currents initiated by the basement morphology, and migration of the Tone River. Due to sea-level lowering from Marine Isotope Stage 5 to LGM, the Nakagawa and Arakawa Valleys were formed under the lowlands. The last deglacial sea-level rise caused the valleys to change from braided river to meandering river, tidal flat, tidal river, and tide-influenced bay environments. During the Holocene sea-level highstand, the tide-influenced bay in the Arakawa Valley was first filled with Tone River sediment input, and after the migration of the main stream of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, the tide-influenced bay in the Nakagawa Valley was filled up. The tide-influenced bay in the Nakagawa Valley at 5 ka can be regarded as a tide-dominated delta, and its regime changed from tide-dominated to river-dominated after the bay filled up at 3.5 ka.